Last month, an international team of scientists published the results of the first-ever DNA sequencing of the entire human microbiome - the colonies of bacteria that live in and on our bodies, sometimes working against us when we're sick, but mostly working with us in harmless and even favorable ways.

The results of their sequencing are staggering. The human body carries more than 100 trillion bacteria - up to five pounds of the tiny single-celled organisms. The mouth alone has several hundred species of bacteria. Each tooth is its own ecosystem.

Together, all of the bacteria in the body would be the size of a large liver, and in many ways, scientists say, the microbiome behaves as another organ in the human body.

Each of our bodies has its own unique microbiome, cultivated from birth and built from our genes and our diet, nurtured by our exposure to a family dog or cat, by how much dirt we ate out of the sandbox and the antibiotics we've taken for ear infections or strep throat.

Bacteria, for more than a hundred years seen only as a bane of human existence - the cause of fatal illnesses and gut-cramping food poisoning - have, over the past decade, increasingly come to be seen as benevolent life partners. Most people will carry the same basic set of bacteria over their lifetime, and while some microbes may cause gingivitis, others may be actively working to keep our gums healthy.

One hundred trillion is represented numerically as 100,000,000,000,000. It's worth taking a moment to contemplate that number:

There are seven billion or so human beings alive today on planet Earth. It would take about 14,285 planets as full of people as Earth is today to add up to 100,000,000,000,000 people.

Our galaxy contains somewhere between one and four hundred billion stars. It would take somewhere between two hundred fifty and one thousand Milky Way galaxies to add up to 100,000,000,000,000 stars.

According to a U. Hawaii calculation (that is, a guesstimate expressed in formula notation), there are 7,500,000,000,000,000,000 grains of sand on all the beaches on Earth. It would take the bacteria resident in the bodies of some 75,000 people -- about one and a third times the population of Santa Cruz, California, judged the best surf town in the U.S. by Surfer Magazine in 2009 -- to add up to the number of grains of sand on beaches planetwide.

Bottom line? One hundred trillion is a big number.

What I find staggering is how this newly-understood scope of the human microbiome impacts any conceivable concept of "self" -- a topic of interest to narcissists, philosophers, and readers of Ayn Rand throughout the ages. What does "I" mean when each of us is a massively populated ecosystem? When each of us is, so to speak, a teeming zoo enclosed in a bag of skin.

Might Decartes have declaimed differently in Discourse on Method if he had understood how many living beings constitute an "individual"? I contain, therefore I am in place of Je pense donc je suis?

George Harrison -- of The Beatles -- didn't wait for the scientific results to come in before seeking to understand life, self, and soul in proper scale. He looked instead to his sitar-plucking pals in India, and sang it out on the album Sgt. Pepper's Lonely Hearts Club Band. I submit for your listening pleasure Within You and Without You, with excerpted lyrics below the embedded video:

And to see you're really only very small,and life flows on within you and without you.[...]We were talkingabout the love that's gone so cold.And the peopleWho gain the world and lose their soul.They don't knowthey can't seeare you one of them?When you've seen beyond yourselfthen you may findpeace of mind is waiting there.And the time will come when you seewe're all one, and life flows on within you and without you.

Leaving numbers aside, and platinum-before-there-was-such-a-thing record albums, check out Carl Zimmer's mind-blowing article, Tending the Body's Microbial Garden in the 18 June 2012 issue of the NY Times. This excerpt, for example:

A number of recent reports shed light on how mothers promote the health of their children by shaping their microbiomes. In a study published last week in the journal PLoS One, Dr. Kjersti Aagaard-Tillery, an obstetrician at Baylor College of Medicine, and her colleagues described the vaginal microbiome in pregnant women. Before she started the study, Dr. Aagaard-Tillery expected this microbiome to be no different from that of women who weren’t pregnant.

“In fact, what we found is the exact opposite,” she said.

Early in the first trimester of pregnancy, she found, the diversity of vaginal bacteria changes significantly. Abundant species become rare, and vice versa.

One of the dominant species in the vagina of a pregnant woman, it turns out, is Lactobacillus johnsonii. It is usually found in the gut, where it produces enzymes that digest milk. It’s an odd species to find proliferating in the vagina, to say the least. Dr. Aagaard-Tillery speculates that changing conditions in the vagina encourage the bacteria to grow. During delivery, a baby will be coated by Lactobacillus johnsonii and ingest some of it. Dr. Aagaard-Tillery suggests that this inoculation prepares the infant to digest breast milk.